Because they provide a sustainable energy resource, the use of solar cells is rapidly expanding. Solar cells can typically be categorized into two types based on the light absorbing material used, i.e., bulk or wafer-based solar cells and thin film solar cells.
Monocrystalline silicon (c-Si), poly- or multi-crystalline silicon (poly-Si or mc-Si) and ribbon silicon are the materials used most commonly in forming the more traditional wafer-based solar cells. Solar cell modules derived from wafer-based solar cells often comprise a series of self-supporting wafers (or cells) that are soldered together. The wafers generally have a thickness of between about 180 and about 240 μm. Such a panel of solar cells is called a solar cell layer and it may further comprise electrical wirings such as cross ribbons connecting the individual cell units and bus bars having one end connected to the cells and the other exiting the module. The solar cell layer is then further laminated to encapsulant layer(s) and protective layer(s) to form a weather resistant module that may be used for up to 25 to 30 years. In general, a solar cell module derived from wafer-based solar cell(s) comprises, in order of position from the front light-receiving side to the back non-light-receiving side: (1) an incident layer, (2) a front encapsulant layer, (3) a solar cell layer, (4) a back encapsulant layer, and (5) a backing layer.
The increasingly important alternative thin film solar cells are commonly formed from materials that include amorphous silicon (a-Si), microcrystalline silicon (μc-Si), cadmium telluride (CdTe), copper indium selenide (CuInSe2 or CIS), copper indium/gallium diselenide (CuInxGa(1−x)Se2 or CIGS), light absorbing dyes, and organic semiconductors. By way of example, thin film solar cells are disclosed in e.g., U.S. Pat. Nos. 5,507,881; 5,512,107; 5,948,176; 5,994,163; 6,040,521; 6,137,048; and 6,258,620 and U.S. Patent Publication Nos. 20070298590; 20070281090; 20070240759; 20070232057; 20070238285; 20070227578; 20070209699; and 20070079866. Thin film solar cells with a typical thickness of less than 2 μm are produced by depositing the semiconductor layers onto a superstrate or substrate formed of glass or a flexible film. During manufacture, it is common to include a laser scribing sequence that enables the adjacent cells to be directly interconnected in series, with no need for further solder connections between cells. As with wafer cells, the solar cell layer may further comprise electrical wirings such as cross ribbons and bus bars. Similarly, the thin film solar cells are further laminated to other encapsulant and protective layers to produce a weather resistant and environmentally robust module. Depending on the sequence in which the multi-layer deposition is carried out, the thin film solar cells may be deposited on a superstrate that ultimately serves as the incident layer in the final module, or the cells may be deposited on a substrate that ends up serving as the backing layer in the final module. Therefore, a solar cell module derived from thin film solar cells may have one of two types of construction. The first type includes, in order of position from the front light-receiving side to the back non-light-receiving side, (1) a solar cell layer comprising a superstrate and a layer of thin film solar cell(s) deposited thereon at the non-light-receiving side, (2) a (back) encapsulant layer, and (3) a backing layer. The second type may include, in order of position from the front light-receiving side to the back non-light-receiving side, (1) an incident layer, (2) a (front) encapsulant layer, (3) a solar cell layer comprising a layer of thin film solar cell(s) deposited on a substrate at the light-receiving side thereof.
The encapsulant layers used in solar cell modules are designed to encapsulate and protect the fragile solar cells. Suitable polymer materials for solar cell encapsulant layers typically possess a combination of characteristics such as high impact resistance, high penetration resistance, good ultraviolet (UV) light resistance, good long term thermal stability, adequate adhesion strength to glass and other rigid polymeric sheets, high moisture resistance, and good long term weatherability. In addition, the front encapsulant layers should be transparent enough to allow sunlight to effectively reach the solar cells to permit the solar cells to generate the highest power output possible. Thus, it is very desirable that the polymer materials utilized in the front encapsulant layers exhibit a combination of low haze and high clarity.
Ionomers are polymers produced by partially or fully replacing the hydrogen atoms of the acid moieties of precursor (also known as “parent”) acid copolymers with ionic moieties. This is generally accomplished by neutralizing the parent acid copolymers, for example copolymers comprising copolymerized units of α-olefins and α,β-ethylenically unsaturated carboxylic acids. Neutralization of the carboxylic acid groups present in such parent or precursor copolymers is generally effected by reaction of the copolymer with a base, e.g. sodium hydroxide or magnesium hydroxide, whereby the hydrogen atoms of the carboxylic acids are replaced by the cations of the base. The ionomers thus formed are ionic, fully or partially neutralized compositions that comprise carboxylate groups having cations derived from reaction of the carboxylic acid with the base. Ionomers are well known in the art and include polymers wherein the cations of the carboxylate groups of the ionomer are metal cations, including alkali metal cations, alkaline earth cations and transition metal cations. Commercially available ionomers include those having sodium, magnesium, aluminum, zinc and iron cations.
The use of ionomer compositions in laminated safety glass as interlayers is known in the art. See, e.g., U.S. Pat. Nos. 3,344,014; 3,762,988; 4,663,228; 4,668,574; 4,799,346; 5,759,698; 5,763,062; 5,895,721; 6,150,028; and 6,432,522, U.S. Patent Publication Nos. 20020155302; 20020155302; 20060182983; 20070092706; 20070122633; 20070289693, and PCT Patent Publication Nos. WO9958334; WO2006057771 and WO2007149082.
In recent years, certain ionomer compositions have also been developed as solar cell encapsulant materials. See, e.g., U.S. Pat. Nos. 5,476,553; 5,478,402; 5,733,382; 5,741,370; 5,762,720; 5,986,203; 6,114,046; 6,187,448; 6,353,042; 6,320,116; and 6,660,930, and U.S. Patent publication Nos. 20030000568 and 20050279401. For example, U.S. Pat. No. 5,476,553 discloses the use, among others, of sodium ionomers such as Surlyn® 1601 resin as an encapsulant material. U.S. Pat. No. 6,114,046 discloses a multi-layer metallocene polyolefin/ionomer laminate structure that can be used as an encapsulant. Various types of ionomers, including sodium and zinc ionomers, are described.
The transparency of ionomer encapsulants currently used as well as those disclosed in the art is not optimum. It would be desirable to have available ionomer compositions of greater transparency so that production of solar cells capable of generating higher power output would be possible.